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Tue, 27 Feb 2024 17:49:48 GMT Received: from smtpav02.dal12v.mail.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 6C3DC58051; Tue, 27 Feb 2024 17:49:46 +0000 (GMT) Received: from smtpav02.dal12v.mail.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 4410658065; Tue, 27 Feb 2024 17:49:42 +0000 (GMT) Received: from [9.43.122.193] (unknown [9.43.122.193]) by smtpav02.dal12v.mail.ibm.com (Postfix) with ESMTP; Tue, 27 Feb 2024 17:49:41 +0000 (GMT) Message-ID: <42d5da16-b0d6-48f6-8da3-356f1717bb06@linux.ibm.com> Date: Tue, 27 Feb 2024 23:19:40 +0530 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 1/2] sched/fair: Add EAS checks before updating overutilized Content-Language: en-US To: Chen Yu Cc: mingo@kernel.org, peterz@infradead.org, vincent.guittot@linaro.org, dietmar.eggemann@arm.com, linux-kernel@vger.kernel.org, nysal@linux.ibm.com, aboorvad@linux.ibm.com, srikar@linux.vnet.ibm.com, vschneid@redhat.com, pierre.gondois@arm.com, morten.rasmussen@arm.com, qyousef@layalina.io References: <20240223150707.410417-1-sshegde@linux.ibm.com> <20240223150707.410417-2-sshegde@linux.ibm.com> From: Shrikanth Hegde In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-TM-AS-GCONF: 00 X-Proofpoint-GUID: y1f_8uyAX7FLqf533c3wyZJaYCxDB-22 X-Proofpoint-ORIG-GUID: vSDXGdv01Xke14sKbEqRdjDgL2SNZtb0 X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.272,Aquarius:18.0.1011,Hydra:6.0.619,FMLib:17.11.176.26 definitions=2024-02-27_05,2024-02-27_01,2023-05-22_02 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 bulkscore=0 lowpriorityscore=0 clxscore=1011 spamscore=0 mlxlogscore=999 suspectscore=0 mlxscore=0 impostorscore=0 adultscore=0 priorityscore=1501 phishscore=0 malwarescore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2311290000 definitions=main-2402270137 On 2/27/24 10:15 PM, Chen Yu wrote: > On 2024-02-23 at 20:37:06 +0530, Shrikanth Hegde wrote: >> Overutilized field of root domain is only used for EAS(energy aware scheduler) >> to decide whether to do regular load balance or EAS aware load balance. It >> is not used if EAS not possible. >> >> Currently enqueue_task_fair and task_tick_fair accesses, sometime updates >> this field. In update_sd_lb_stats it is updated often. >> Which causes cache contention due to load/store tearing and burns >> a lot of cycles. > > Looks like a typical cache false sharing: CPU1 updates the rd->overutilized, > which invalid the cache line when CPU2 access adjacent rd->overload. > This changes looks good to me, just some minor questions: Thanks for taking a look and reviewing it. > >> Hence add EAS check before updating this field. >> EAS check is optimized at compile time or it is static branch. >> Hence it shouldn't cost much. >> >> With the patch, both enqueue_task_fair and newidle_balance don't show >> up as hot routines in perf profile. >> >> 6.8-rc4: >> 7.18% swapper [kernel.vmlinux] [k] enqueue_task_fair >> 6.78% s [kernel.vmlinux] [k] newidle_balance >> +patch: >> 0.14% swapper [kernel.vmlinux] [k] enqueue_task_fair >> 0.00% swapper [kernel.vmlinux] [k] newidle_balance >> >> While here, Fix updating overutilized as either SG_OVERUTILIZED or 0 >> instead. Current code can make it 0, 1 or 2. This shouldn't alter the >> functionality. > > Just wonder where 1 comes from? In current code we either write SG_OVERUTILIZED > or sg_status & SG_OVERUTILIZED. Thanks for catching this, Silly mistake. Because of if conditions around I wrongly thought it would be 1. I will correct that and send a next version soon. > >> >> Fixes: 2802bf3cd936 ("sched/fair: Add over-utilization/tipping point indicator") >> Signed-off-by: Shrikanth Hegde >> --- >> kernel/sched/fair.c | 36 +++++++++++++++++++++++++----------- >> 1 file changed, 25 insertions(+), 11 deletions(-) >> >> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c >> index 8e30e2bb77a0..9529d9ef2c5b 100644 >> --- a/kernel/sched/fair.c >> +++ b/kernel/sched/fair.c >> @@ -6670,15 +6670,30 @@ static inline bool cpu_overutilized(int cpu) >> return !util_fits_cpu(cpu_util_cfs(cpu), rq_util_min, rq_util_max, cpu); >> } >> >> -static inline void update_overutilized_status(struct rq *rq) >> +static inline void update_rd_overutilized_status(struct root_domain *rd, >> + int status) >> { >> - if (!READ_ONCE(rq->rd->overutilized) && cpu_overutilized(rq->cpu)) { >> - WRITE_ONCE(rq->rd->overutilized, SG_OVERUTILIZED); >> - trace_sched_overutilized_tp(rq->rd, SG_OVERUTILIZED); >> + if (sched_energy_enabled()) { >> + WRITE_ONCE(rd->overutilized, status); >> + trace_sched_overutilized_tp(rd, !!status); > > Is this !!status intentional? The original one is SG_OVERUTILIZED = 2, > now it is either 0 or 1. > Yes. this is intentional. To convert into to bool. The tracepoint hook currently defines the second argument as bool. include/trace/events/sched.h DECLARE_TRACE(sched_overutilized_tp, TP_PROTO(struct root_domain *rd, bool overutilized), TP_ARGS(rd, overutilized)); > thanks, > Chenyu